Mesh appears normal in solid mode but looks weird in textured mode
The most likely cause would be the normals, that is which way the polygon is facing, the outer side is the one that has the texture drawn on it. In edit mode select all and press CtrlN. If that makes it worse try CtrlShiftN to flip them the other way.

Worst case scenario you can select individual faces and flip the normals using Mesh->Normals->Flip (Flip normals is also available in the specials menu W)